Role of functionalized transition-metal coated W tips in STM imaging: Application to epitaxial graphene on SiC(0001)

Scanning tunneling microscopy using W tips functionalized with transition-metal (Cr, Fe) coatings is demonstrated to facilitate the imaging of states within a few millielectron volts (meV) of the Fermi level for epitaxial graphene on SiC(0001), which are not accessible with bare W tips. First-principles modeling of these probe tips as pyramidal structures on W(110) indicates that an apex atom is stable for the Cr/W(110) tip but not for the Fe/W(110) or W/W(110) tips. This difference in their atomic structures, together with the variation in the extent of the 3d and 5d orbitals, is found to be responsible for the capability of Cr and Fe functionalized tips to selectively image the complex electronic properties of epitaxial graphene on SiC(0001).

Figure 1

STM images of the epitaxial graphene on 6H-SiC(0001) taken with a W tip. (a) Filled state (sample bias V${}_{\mathrm{s}}$ = −1.5 V, tunneling current I${}_{\mathrm{t}}$ = 2.0 nA, size = 17 × 17 nm${}^2$). (b) Empty state (V${}_{\mathrm{s}}$ = +1.5 V, I${}_{\mathrm{t}}$ = 2.0 nA, size = 17 × 17 nm${}^2$).

Figure 2

STM images taken with an Fe-coated W tip at (a) V${}_{\mathrm{s}}$ = −1.0 V, I${}_{\mathrm{t}}$ = 0.5 nA; and (b) V${}_{\mathrm{s}}$ = −0.5 V, I${}_{\mathrm{t}}$ = 0.4 nA. Image size is 16 × 16 nm${}^2$ for both.

Figure 3

STM images taken with an Fe-coated W tip at (a) V${}_{\mathrm{s}}$ = −0.1 V and (b) V${}_{\mathrm{s}}$ = +0.1 V; I${}_{\mathrm{t}}$ = 0.2 nA. Image size is 15 × 10 nm${}^2$ for both. (c) STM image taken with a Cr-coated W tip at V${}_{\mathrm{s}}$ = −5.0 meV, I${}_{\mathrm{t}}$ = 0.7 nA, size = 15 × 15 nm${}^2$.

Figure 4

Selective imaging of the first- and second-layer graphene with an Fe/Cr/W tip (Cr is deposited first to improve adhesion) at (a) V${}_{\mathrm{s}}$ = −0.062 V, and (b) V${}_{\mathrm{s}}$ = −0.62 V; I${}_{\mathrm{t}}$ = 4.9 nA and image size is 17 × 17 nm${}^2$ for both. (c) Selective imaging of the interfacial layer and first graphene layer; the sample bias is changed from −0.051 V (top) to −0.51 V (bottom) where indicated. I${}_{\mathrm{t}}$ = 5.3 nA and size = 17 × 17 nm${}^2$.

Figure 5

(a) Top and side view of a ball-and-stick model used in the calculations. The apex (red) and base (yellow) atoms of the pyramid are labeled; the first substrate layer (orange) is Fe (Cr) in the case of the Fe/W (Cr/W) tip. (W substrate atoms are light blue.) Calculated LDOS (blue [orange] for majority [minority]) are given for the base atoms of (b) W/W(110) and (c) Fe/W(110), and (d) of the apex atom for Cr/W(110).

Figure 6

Density of the tunneling states in the separation plane for the Cr/W, Fe/W, and bare W tips corresponding to states around the Fermi level and 0.25 eV above and below E${}_{\mathrm{F}}$, given relative to the average density in the plane (see scale). The height of the separation plane corresponding to the same constant current is given in each panel. The positions of atoms in the pyramid (substrate) are indicated by dark (light) dots.

Figure 7

Density of the tunneling states (relative to the average density) in the separation plane for the Fe/W and bare W tips for filled and empty states corresponding to bias voltages of 0.5 and 1.0 V (see scale). The height of the separation plane corresponding to a constant current is given in each panel.

Figure 8

Spin-resolved tunneling states (relative to the total average density; see scale) in the separation plane for Cr/W and Fe/W tips for different bias voltages. The net spin polarization in the plane is given at the bottom of each column.